Application of LD7552+TL494 integrated board in LCD display

People who are familiar with power supplies know that the integrated board of LD7552 + TL494 is used in various brands of machines. This article introduces a universal 2-lamp integrated board for 18.5-inch to 23.6-inch.

LD7552 is a high-performance PWM controller launched by Tongjia Technology. It has two package forms, SOP-8 and DIP-8, and the pin functions are the same. The LD7552 is mainly composed of undervoltage lockout circuit, working oscillator and green mode oscillator, leading edge blanking circuit, slope compensation circuit, final drive circuit, etc. When the supply voltage is lower than 11.4V, the undervoltage protection mode is activated. Only when the supply voltage returns to above 16V again, the chip will start to output the excitation signal again. There are two oscillators inside the LD7552, one is the normal working oscillator and the other is the green mode oscillator. The green mode oscillator does not start in normal operation to ensure the stability of the output frequency and waveform. In standby or no-load conditions, the green mode oscillator starts to reduce the switching frequency, effectively reduce power consumption, and reduce temperature. It can ensure that the system power consumption in standby state is less than 0.3W, which meets the requirements of European energy-saving standards.

Figure 1 is the internal structure of LD7552; Figure 2 is the actual application principle diagram of LD7552 in a 2-lamp power supply high voltage integrated board. From this figure, we can see that the difference between LD7552 and SG6841 is that the 5th pin is empty, and the resistance deviation of the external resistor of the 4th pin is large. In addition, on the actual circuit board, the 3rd and 7th pins of LD7552 are connected together. When replacing the two, the resistance value of the switch tube current sampling resistor must be adjusted according to the specific circuit.

Table 1 shows the pin functions and measured data of LD7552. TL494 is a well-known bipolar linear integrated circuit, which contains all the control circuits of pulse width modulation switching power supply. It has a built-in linear sawtooth oscillator, and the oscillation frequency can be adjusted by an external resistor and a capacitor. It was widely used in ATX power supplies and chargers in the early days. Because of its low cost and mature technology, it is now widely used in liquid crystal displays. Its shadow can be seen in both the power supply part and the high-voltage part, especially the high-voltage part. The internal circuit consists of a reference voltage generation circuit, an oscillation circuit, an interval adjustment circuit, two error amplifiers, a pulse width modulation comparator and an output circuit. The built-in 5.0v reference voltage source can provide a load current of up to 10mA when using an external bias circuit. The maximum working voltage is 42V. There are two packaging forms, DIP-16 and SOP-16. Most of the liquid crystal displays use SOP packaging.

Figure 2 TL494 internal structure diagram

In this circuit, Q204 and Q207 form an enabling control circuit, and the models are PDTC144WK and PDTA144WK respectively. These are two band-stop transistors with different polarities. They cannot be replaced with ordinary transistors after being damaged, otherwise the circuit will not work properly. PMBS3906 and PMBS3904 used in the circuit can be replaced with common 8050, 8550 or 2N5401, 5551, just pay attention to the polarity and pin function. The final power output uses a SOP-8 packaged power switch field effect tube AM9945N, which is a chip that simultaneously encapsulates two N-channel field effect tubes internally, with basic parameters of 60V/3.6A/2.1W. After being damaged, they can be replaced with FDS9945, STM6930A, SI4946 and other models.

Table 2 shows the pin functions and measured data of TL494. Figures 5 and 6 show the actual power supply high voltage integrated board and test diagram using this combination scheme. The DC output is 12V and 5V, which can be used in 18.5-inch to 23.6-inch 2-lamp LCD displays.

Figure 6 Test diagram of the power supply high voltage integrated board using this combination solution